1. Field of the Invention
The present invention relates to a method for manufacturing a piston for an internal combustion engine, and in particular to a method for manufacturing a piston for an internal combustion engine which forms an oil-cooling gallery portion by a soluble core.
2. Description of the Prior Art
A conventional method for manufacturing a piston of the foregoing type is disclosed, for example, in Japanese Application Laid-Open Publication No. 60-166158 published on Aug. 29, 1985. Therein, the circumference of a top-ring groove is reinforced by an aggregate of inorganic fiber or a porous metal and a soluble core is adhered to an inner side of the aggregate of the inorganic fiber as a strength or reinforcing member in order to form an oil-cooling gallery portion near the top-ring groove. The lower part of the soluble core has pinholes and the upper part of the punch mold has three poles. A pin is formed at the top of the poles and is inserted in the pinhole of the soluble core. In this condition, the soluble core is held in the punch mold and is inserted in the casting mold. After the casting, the soluble core is melted and the oil-cooling gallery is formed.
In the prior art, however, since the soluble core is held in the punch mold and the aggregate of the inorganic fiber is fragile, the aggregate of the inorganic fiber sometimes is broken upon the casting and the aggregate of the inorganic fiber adheres to the soluble core. Therefore, the quality of the piston deteriorates, and X-ray inspection of the piston is necessary to determine flaws. The above drawback can be prevented by the use of a porous metal. However, the porous metal is expensive and, therefore, the manufacturing cost of the piston is increased.